SiC (silicon carbide) is expected as a material for a next-generation semiconductor device. SiC have excellent physical properties in comparison with Si (silicon). A band gap is three times, breakdown electric field strength is approximately ten times, and thermal conductivity is approximately three times. A low loss semiconductor device which can operate at a high temperature can be realized by using such properties.
However, for example, in the case where a metal insulator semiconductor field effect transistor (MISFET) is formed by using SiC, an interface state density between a semiconductor and a gate insulating layer increases in comparison with a case where Si is used. Therefore, there is a problem that mobility of electrical charges is decreased, and on-resistance of the MISFET is increased.
To solve the above problem, for example, there is a method in which an element of such as N (nitrogen) or P (phosphorus) to terminate an interface state is introduced at an interface between SiC and an insulating film. If this method is used, N (nitrogen) and P (phosphorus) function as an n-type dopant, and a threshold voltage of an n-channel type MISFET may fall down.
In order to prevent from causing malfunction of a SiC-MOSFET, the threshold voltage at least equal to or greater than 3 V is required at an operation temperature (for example, 200° C.), and preferably the threshold voltage is equal to or greater than 5 V. Because the threshold voltage may fall down to around 1 V, it is difficult to keep the required threshold voltage with termination using nitrogen or phosphorus.